In recent years, arts of transmitting image information in the form of electric signals, and storing it at the reception points or reproducing it on CRT have been developed remarkably. In parallel with development of these arts, the demand for the production of hard copies from the transmitted image information has been increasing, and in response thereto various means of obtaining hard copies have been proposed. For instance, electrophotographic methods and heat-sensitive transfer methods utilizing sublimation of dyes have been applied. However, most of hard copies obtained using such methods are poor in image quality. As for the color hard copies in particular, the quality thereof has no comparison with that of prints using color papers on the market. In addition, there is a method of obtaining hard copies from digital image information by the use of silver salt photography, wherein CRT is used for exposure in obtaining printed images. As the situation now stands, however, satisfactory image quality is not yet obtained for a reason that CRT beams have a large spot area, and so on. As a system which can offer hard copies of high quality, on the other hand, there are on the market "Pictrography" , trade name, produced by Fuji Photo Film Co., Ltd., and the like, which utilizes the combination of an image formation method comprising processes of the heat development of silver halide and the diffusion of dyes with an exposure method comprising scanning exposure with LED. However, such a system has a price problem.
Meanwhile, rapid and cheap service of extremely high quality prints is now performed with relatively ease owing to improvements in silver halide photosensitive materials and the progress of compact, simple and rapid development system (e.g., a minilab system). After the analogy of this, the demand for materials which can provide hard copies of image information at a low price, enable simple and rapid processing, and ensure steady acquisition of their properties and high image quality is now strong and growing.
In the method of obtaining hard copies from electric signals, a scanning exposure system in which exposure is performed as image information is picked out successively is generally adopted. Therefore, sensitive materials suitable for this system are required.
As for the scanning exposure applicable to image formation, there is a so-called scanner system. Various kinds of recording apparatuses utilizing the scanner system are on the market. As a recording light source installed in such recording apparatuses, a glow lamp, a xenon lamp, a mercury lamp, a tungsten lamp, a light emitting diode and so on have so far been used. However, all of these light sources are weak in generating power and have a short span of life, and so they are undesirable from the practical point of view. As light sources which enable obviation of the defects of such light sources, devices for emission of coherent light, such as gas laser, e.g., He-Ne laser, argon laser, He-Cd laser, etc., and semiconductor laser, are usable. In practice, there are scanners using those laser devices as a light source.
Although it is high in generating power, gas laser has a defect that it requires a large-sized expensive device for generating laser beams and a modulator, and so on.
in comparison with gas laser, semiconductor laser has many advantages. For instance, semiconductor laser devices are small in size and low in price, laser beams generated therefrom can be modulated with ease, and a life-span thereof is longer than that of gas laser devices. Since wavelengths of laser beams emitted from semiconductor devices are mainly within a range of from red to infrared region, it is required of photosensitive materials to have high spectral sensitivities at the wavelengths ranging from red to infrared region.
However, most of sensitizing dyes which can confer spectral sensitivities on photosensitive materials within the wavelength range from red to infrared region are those of monomer band type, and so the wavelength dependence of the spectral sensitivities gained is generally indistinct. Supposing a full color photosensitive material for exposure to laser beams (exposure wavelengths: .lambda.a, .lambda.b and .lambda.c) is designed using those sensitizing dyes (for example, so as to form a yellow color by exposure to a laser beam of .lambda.a, a magenta color by exposure to a laser beam of .lambda.b and a cyan color by exposure to a laser beam of .lambda.c), unnecessary colors will be formed in high density areas of the color intended to be formed because light-sensitive layers, other than the proper light-sensitive layer to form a color by exposure to a given laser beam, will also have some sensitivity to said laser beam owing to broad distribution of spectral sensitivities given thereto by sensitizing dyes. (More specifically, if a magenta color is formed by exposure to a laser beam of .lambda.b, yellow and cyan colors also will come to be formed as the quantity of beam is increased in said exposure.) This supposition suggests that the use of sensitizing dyes of monomer band type results in unsatisfactory color separation.
In general, conventional color photosensitive materials have been designed so as to form a yellow color by exposure to blue light, a magenta color by exposure to green light and a cyan color by exposure to red light. Therein, J-band type sensitizing dyes sharp in distribution of spectral sensitivities provided thereby have been used as sensitizing dyes (for blue-sensitive and green-sensitive layers) and wavelength regions of exposure lights have been kept apart from each other (by rendering constituent layers green-sensitive and red-sensitive, respectively), whereby having avoided the foregoing problem. However, it is the present situation that J-band type sensitizing dyes which can produce desired effects in the range from red to infrared region are little known. Moreover, there is a limitation on wavelengths of laser beams which can be used stably. As a result of it, a wavelength difference between every desirable pair of exposure beams becomes at most 80 nm or so. Therefore, the insufficiency in color separation, which may be left out of consideration in case of conventional photosensitive materials, becomes a grave problem in designing full color photosensitive materials utilizing semiconductor laser beams as light source for exposure.
As means of improving color separation in the range from red to infrared region, there can be thought up various ideas of, e.g., (1) making an ample difference in sensitivity between each pair of light-sensitive layers, (2) designing a photosensitive material so as to produce contrasty image, (3) using sensitizing dyes capable of providing spectral sensitivities distributed as narrowly as possible, and (4) providing a filter layer between light-sensitive layers which have a need for color separation, and so inhibiting as completely as possible the light for sensitization of the upper layer from reaching the lower layer.
As for the means (1), it is a general measure and common-sense to those designing photographic materials. However, adoption of this means is attended by difficulties in designing emulsions and selecting laser devices.
As for the means (2), it has an advantage in color separation, but imparting a contrasty characteristic to a photosensitive material signifies that a slight fluctuation of the quantity of light results in a great change of density. Accordingly, this means undergoes a great influence of fluctuation, e.g., in an exposure apparatus, and so the system control becomes very difficult.
As for the means (3), it is a concrete measure to use J-band sensitizing dyes. However, as previously described, J-band type sensitizing dyes capable of exhibiting desirable effects in the infrared region have scarcely been found. On the other hand, it is known that infrared sensitizing dyes of monomer type can provide narrowly distributed spectral sensitivities by assuming rigidly selected structures, as disclosed in JP-A-03-20730 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), European Patents 0420011 and 0420012. However, effects produced by some of such dyes are slight, and some others are inferior in stability or exert adverse effects on photographic properties.
As for the means (4), there can be taken such a measure as to provide a nondiffusible filter layer between two light-sensitive layers in order to reduce as sharply as possible the rays of light, to which the upper layer is sensitive, in the quantity to reach the lower layer, as disclosed in U.S. Pat. No. 4,619,892. However, such a nondiffusible filter layer tends to cause color stain after photographic processing, which brings on a serious problem to hard copies using a reflecting support. This problem becomes more serious the more sharply the development time is reduced for the purpose of rapid production of hard copies.
Therefore, it can be said that development of photosensitive materials well-suited for the exposure using semiconductor laser and excellent in color separation is quite difficult.
Moreover, the exposure using rays of light having high density and long wavelengths, such as laser beams of wavelengths ranging from red to infrared region, is attended by considerable spread of rays arising from halation and irradiation phenomena, and so it causes great deterioration in resolution. Accordingly, water-soluble dyes are used for preventing the rays from spreading through the photosensitive material and for heightening the sharpness. In general, it has so far been carried out to use water-soluble dyes for the purpose of preventing silver halide photographic materials from suffering irradiation. For instance, JP-A-02-157749 discloses a color photosensitive material which has at least two light-sensitive layers sensitized spectrally so as to respond to laser beams of wavelengths longer than 670 nm and is colored with a coloring material which can be decolored during photographic processing. As a material which has both colorability and decolorability, oxonol dyes, hemioxonol dyes, merocyanine dyes, cyanine dyes and the like are known generally. While it was believed that any of those dyes can be used for prevention of irradiation as far as their absorption wavelengths are in a desirable range and they hardly generate color stain, it has turned out that color separation was aggravated by increasing an amount of the dye added with the invention of complete prevention of irradiation. Thus, the color separation problem in color photosensitive materials particularly designed so as to have an aptitude with the exposure to at least two kinds of semiconductor laser beams, which is intrinsically difficult, is made much more serious by linking up with the aggravation of color separation caused by water-soluble dyes.
At the present time, semiconductor laser devices and light emission diodes enable the use of laser beams having wavelengths longer than about 570 nm. In particular, semiconductor devices which can emit laser beams of wavelengths no shorter than 670 nm are already put to practical use.